The substantial viscosity of the hydrocarbons present in certain deposits (heavy oils) poses considerable extraction problems. In such cases, it is generally necessary to decrease the viscosity of (fluidify) heavy oils so as to make them more mobile and therefore be able to extract them. This is particularly important for the exploitation of bituminous sands or shales. Many techniques have already been proposed to that end, in particular “SAGD” (steam-assisted gravity drainage), which consists of injecting steam into the deposit, heating it by heat conduction (for example using electric resistances) or in-situ combustion, which consists of injecting an oxidizing agent, generally air, through injection wells and initiating a combustion within the deposit, so as to develop combustion fronts from air injection wells and towards the production wells.
Another technique that has been proposed consists of proceeding with in situ electromagnetic heating of the reservoir. A first category of in situ electromagnetic heating of the reservoir is that of heating by electromagnetic radiation (i.e. radiofrequency or microwave) using an antenna arranged in the reservoir. Document WO 2007/147053 describes an example of such a system: a radiofrequency generator is placed on the surface; the energy produced is irradiated via a radiofrequency antenna positioned in a specific horizontal or vertical well. The production well, part of which is horizontal, is situated under the radiofrequency antenna.
A second category of in situ electromagnetic heating of the reservoir is that of induction heating. For example, document WO 2008/098850 describes, in one particular embodiment, an injection well geometry passing through the reservoir and imposing a circulation of electric current caused in the reservoir. The injection well also has a steam injection function. A high-frequency generator provides the electrical power necessary for the induction. The two terminals of the generator are connected to the two ends of the injection well, which thus heats the reservoir by induction. The injection well therefore goes up to the surface, the two ends of the injection well then necessarily being connected to the generator. The well then has a particular geometry, of the U-well type. In other cases, the electric circuit is formed by the injection well on one hand (connected to a terminal of the generator), and an electrode installed in a pocket of saltwater on the other hand (connected to the other terminal of the generator). In still another case, the heating for the reservoir is of the resistive type, an electric circuit being established between two remote wells, situated on either side of a deposit to be heated.
The drilling geometry necessary to implement induction heating for these two types of architecture would be extremely complex to produce. Moreover, in these two architectures, the injection tube heats the reservoir by induction over the entire length thereof, therefore including in its vertical portion. Substantial energy losses occur at the edges of the conductors, in the overburden.
Document WO 2009/027273 describes a method for injecting water in the reservoir, the water being vaporized by electric heating in the reservoir. For example, the water injection well and the production well can serve as electrodes. Document WO 2009/027262 describes the use of at least one additional pipe electrically connected to the injection well in order to inductively heat the zone situated between the additional pipe and the injection well.
Document WO 2009/027305 describes a plant for heating a hydrocarbon reservoir comprising an outside alternator providing the electrical power serving to power a driving circuit. The magnetic field causes currents in the reservoir, and brings about the heating thereof. One particular conductor, of the Litz cable type, is used in order to proceed with in situ inductive heating. This Litz cable comprises several conductors aligned to facilitate the passage of the current. The strong impedance thus generated at a high frequency is offset by the introduction of serial capacitances, in order to avoid overvoltages. The cable forms a loop in the reservoir, its two ends being connected to a surface generator. This system has the drawback of only working for a single determined electrical frequency, which poses a problem since the frequency must ideally adapt to the nature of the reservoir and the evolution thereof. In other words, this system is not very efficient at the beginning and end of production and involves slow preheating and very good knowledge of the reservoir from the outset.
Moreover, in the main embodiment, the conductors are placed at the same depth in the reservoir, next to each other, at a given distance. Thus, the magnetic radiation given off by one conductor is cancelled by the other conductor. Although such a geometry makes it possible to avoid energy losses in the overburden, it does however require that the conductors be spaced away from each other at the reservoir, to allow the emission of electromagnetic energy and to ensure in fine the heating of the reservoir. This drilling geometry is extremely complex to implement. All of the systems described above have the drawback of being oftentimes heavy and complex to implement. Moreover, these systems are only suited to a very particular type of electromagnetic heating, whether by radiation (at the highest frequencies) or induction (at the lowest frequencies), or are even only suited to a very specific frequency.
There is therefore a need for a system for the electromagnetic heating of an underground formation that is easier to implement and more flexible. In particular, there is a need for a system for electromagnetic heating of an underground formation that can operate by radiation as well as by induction of capacitive currents, in a wide range of frequencies, that can adapt easily to all types of underground formation.